applicatie note

Measurement of Gold Coatings in the Watch Industry

In the medium price segment, “gold” watches are generally made of stainless steel coated with 20 microns of an 18ct Au alloy. The gold plating should serve decorative purposes and at the same time being durable. This expensive coating must be controlled by precise measurements to guarantee performance while limiting cost.

A minimum thickness of 20 microns of 18ct Au alloy is required to ensure the longevity of the gold plating, as the coated parts include the case, the bracelet and the crown – all of which are subject to wear.

Usually, Au coatings are measured using the well established, non-destructive X-ray fluorescence method (XRF). However, the maximum coating thickness determinable with XRF is 10 microns of 18ct Au alloy, necessitating a different non-destructive method: Beta backscattering. The maximum coating thickness measurable with Beta backscattering depends on the radiation source. For watch applications, Strontium is the right choice.

Fig 1: Measurement principle of the beta backscattering method.

Using a Strontium radiation source, beta particles are emitted with high energy (2.27 MeV) to the specimen. The backscattered beta particles from the specimen are then detected by a Geiger-Müller counter. The software of the FISCHERSCOPE® MMS® PC2 calculates the coating thickness from the irradience. A Strontium radiation source allows Au coatings of up to 35 µm to be measured with this method. The size of the measurement spot is determined exactly by the colli-mator used, typically 0.6 x 1.2 mm for this application. Additional sizes are available and collimaters can be changed out easily for other purposes.

The FISCHERSCOPE® MMS® PC2 multi-measurement system, with its integrated Windows™ CE and network capability, features a large, high-resolution color touchscreen as well as LAN and USB ports. It is perfect for straightforward and nondestructive high-precision coating thickness measurements, as required by the watch industry.

The thickness of 20 µm 18ct Au layers on plated watch parts can be measured non-destructively and easily using the beta backscattering method and the FISCHERSCOPE® MMS® PC2 BETASCOPE®. For further information, your local FISCHER partner will be glad to assist you.

Thickness of protective coatings on wristwatch dials

In today's hectic world, there are few people who can do without a wristwatch. The importance of this "cultural companion" has also changed in recent decades. For some, it just tells the time, helping its wearer to structure daily routines. For others, however, it is jewelery made of precious metal, some even set with diamonds and other gemstones. But what most of them have in common is a dial: a signature feature of any watch, and a very delicate part exposed to significant stressors.

In Switzerland – the land of clocks and watchmaking - the production of this component is taken very seriously. Basically, a clock face is made from a metal disk of ferrous alloys, in which the various recesses for numbers, hands, date window, etc. are stamped according to designer specifications. They are then often plated with gold, silver, copper or palladium, depending on the effect desired.

Fig.1: Dial with coating of precious metal

Because it is worn directly on the wrist, a watch is continuously exposed to a wide range of temperature stresses, for example, abrupt changes between body heat and cold outside temperatures in winter, the shock of being dipped into the pool, or baking in the sun – to name just a few. So that the beautiful metal plating retains its brilliant polish without surface discoloration (especially silver), the finished coating is sealed with a thin layer of varnish. Usually around 10-20 microns thick, this protective top-coat is applied by hand with a spray gun, covering the many fine edges of the recesses with the airtight and moisture-proof fixative, thus preventing oxidation and tarnish.

Although in principle a challenging measurement task, the thickness of this lacquer over thin metallic coatings can be determined precisely using the eddy current probe FTA3.3-5.6HF from FISCHER, along with the handheld instrument ISOSCOPE® FMP30. A further aid to improve the measurement accuracy and ease is the support stand V12 BASE. The measurement sequence is then straightforward for the operator: The probe is placed on the dial with the assistance of the support stand, which enforces level and even positioning of the probe. Because operator influence is thus minimized, extremely uniform measurements can be executed, ensuring high repeatability – the standard deviation can be even below hundred nanometers for measuring the thin layer of varnish.

Thin layers of lacquer protecting the dials of valuable watches can be easily and precisely measured using the combination of ISOSCOPE® FMP30, probe FTA3.3-5.6-HF and support stand V12 BASE. For more information, please contact your local FISCHER representative.